ClangCodeModel: Add experimental clangd support

If the user has enabled clangd (default is off), we start up one instance
per project when it is opened/changed (including build config switches),
and trigger background indexing.
So far, the index is used to provide results for locators and "Find
Usages".
Per-document functionality such as semantic highlighting and completion
is still provided by libclang.

Change-Id: I12532fca1b9c6278baab560e7238cba6189cde9f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangrefactoringengine.cpp

@@ -26,6 +26,12 @@
 #include "clangrefactoringengine.h"
 #include "clangeditordocumentprocessor.h"
 
+#include "clangmodelmanagersupport.h"
+
+#include <cpptools/cppmodelmanager.h>
+#include <languageclient/client.h>
+#include <languageclient/languageclientsymbolsupport.h>
+#include <projectexplorer/session.h>
 #include <utils/textutils.h>
 #include <utils/qtcassert.h>
 
@@ -80,5 +86,27 @@ void RefactoringEngine::startLocalRenaming(const CppTools::CursorInEditor &data,
     m_watcher->setFuture(cursorFuture);
 }
 
+void RefactoringEngine::findUsages(const CppTools::CursorInEditor &cursor,
+                                   CppTools::UsagesCallback &&callback) const
+{
+    ProjectExplorer::Project * const project
+            = ProjectExplorer::SessionManager::projectForFile(cursor.filePath());
+    LanguageClient::Client * const client
+            = ClangModelManagerSupport::instance()->clientForProject(project);
+    if (!client || client->state() != LanguageClient::Client::Initialized) {
+        // TODO: Also forward to built-in if index is not ready.
+        //       This requires us to keep track of workDone status in the client.
+        //       Related: Also allow to override the server string for progress info
+        CppTools::CppModelManager::builtinRefactoringEngine()
+                ->findUsages(cursor, std::move(callback));
+        return;
+    }
+    // TODO: We want to keep our "access type info" feature.
+    //       Check whether we can support it using clang 12's textDocument/ast request
+    if (!client->documentOpen(cursor.textDocument()))
+        client->openDocument(cursor.textDocument()); // TODO: Just a workaround
+    client->symbolSupport().findUsages(cursor.textDocument(), cursor.cursor());
+}
+
 } // namespace Internal
 } // namespace ClangCodeModel